This invention relates to polymer devices, for instance transistors that comprise a semiconductive polymer material.
There has been extensive work on transistors made with organic materials. Insulated gate field effect transistors (FETs) have been made with polymer semiconductors deposited by solution processing of either the polymer itself or a precursor to form a layer of the final polymer. FIG. 1 shows the general structure of such a device. Under the semiconductor polymer layer 1 are two spaced apart metallic electrodes, the drain electrode 2 and the source electrode 3 of the transistor. Below them are a layer 4 of Si/SiO2 and a metallic gate electrode 5. The device acts as a switch because current flow between the source and drain electrodes is greatly increased when a bias is applied to the gate electrode. One such device, in which the semiconductor polymer is regioregular poly-hexylthiophene (P3HT), is described in more detail in Z. Bao et al., Appl. Phys. Lett. 69, 4108 (1996).
Devices of this type have several problems (see A. R. Brown et al., Science 270, 972 (1995)). First, the through-current from the source to the drain is low because the electronic carrier mobility xcexc is typically in the range from 10xe2x88x924 to 10xe2x88x926 cm2/Vs. (See J. H. Burroughes et al., Nature 335, 137 (1988) and A. R. Brown et al., Synthetic Metals 88, 37 (1997)). Most solution-processed polymers have a disordered structure, and it is believed that in these systems the carrier mobility is limited by variable-range hopping between polymer chains. This low mobility rules out such transistors for general current-supply applications. Second, the on-off ratio, i.e. the ratio between the through-current in the on and off states, is poor: less than 104 for example. Up to now a polymer transistor with a performance comparable to that of inorganic amorphous silicon transistors has not been demonstrated. As a consequence a preferred approach has been to use molecular (or oligomer) organic materials instead of polymers. Molecular devices tend to have improved electrical performance but have severe process shortcomings. First, the molecules are generally deposited by vacuum sublimation, typically at substrate temperatures around 100-200xc2x0 C. This rules out the use of such molecular materials on heat-sensitive substrates. Second, the molecular materials are generally not robust; there are serious concerns about the effect of cracks and microcracks in highly crystalline sublimed molecular films, in particular if deposited on flexible plastic substrates. Third, molecular devices are highly sensitive to subsequent processing steps. Attempts to post-process sublimed molecular films, for example to deposit subsequent layers on top of the sublimed films for multilayer integrated devices, have generally resulted in greatly reduced performance of the buried FETs.
According to a first aspect of the present invention there is provided an integrated circuit device comprising: a current drive switching element having an input electrode, an output electrode, a switchable region comprising a semiconductive polymer material electrically coupled between the input electrode and the output electrode, and a control electrode electrically coupled to the switchable region so as to allow the application of a bias to the control electrode to vary the flow of current through the switchable region between the input electrode and the output electrode; and a second circuit element, integrated with the switching element, and electrically coupled with the output electrode of the switching element for receiving a drive current from the switching element.
According to a second aspect of the present invention there is provided a method for forming an electronic device having a region comprising a semiconductive polymer material, the method comprising depositing the semiconductive polymer by a process which promotes ordering in the deposited polymer. The electronic device according to this aspect of the invention may suitably be a switching element, for example of the type as set out above in relation to the first aspect of the invention.
According to a third aspect of the present invention there is provided an integrated circuit device comprising: a switching element having an input electrode, an output electrode, a switchable region comprising a semiconductive polymer material electrically coupled between the input electrode and the output electrode, and a control electrode electrically coupled to the switchable region so as to allow the application of a bias to the control electrode to vary the flow of current through the switchable region between the input electrode and the output electrode; and an electro-optical circuit element, integrated with the switching element, and electrically coupled to one of the electrodes of the switching element.
The semiconductive polymer may, for instance, be a conjugated polymer (see, for example, PCT/WO90/13148, the contents of which are incorporated herein by reference) or an xe2x80x9cintermolecularxe2x80x9d semiconducting polymer like poly-vinylcarbazole (PVK) containing short conjugated segments connected by non-conjugated segments.
An insulating layer may be deposited directly or indirectly on top of the electronic device. Preferably this does not substantially degrade the performance of the device. A second circuit element (as in the first aspect of the invention) may also be formed, and is preferably integrated with the said electronic device.
The second circuit element (or the opto-electrical element of the third aspect of the invention) is preferably an element that stores or consumes (preferably significant) electrical energy, e.g. by converting current to an electrical or optoelectrical signal, or an element that converts an optical signal into an electrical signal, e.g. a voltage or a current. The second circuit element is preferably not a switching element. The second circuit element is suitably capable of emitting or detecting light and/or varying the transmission of light through itself. Examples include light-emissive devices, photovoltaic devices and devices such as liquid crystal devices. The device may suitably emit or detect an optical signal, it may be a display device and/or form part of a visual display. The second circuit element preferably requires a significant drive current for its operation.
Where the second circuit element is a light-emissive element it is preferred that it comprises one or more light-emissive organic materials. The or each light-emissive organic material may be a polymer material, preferably a conjugated or partially conjugated polymer material. Suitable materials include poly-phenylene-vinylene (PPV), poly(2-methoxy-5(2xe2x80x2-ethyl)hexyloxyphenylene-vinylene) (MEHPPV), PPV-derivatives (e.g. di-alkoxy or di-alkyl derivatives), polyfluorenes and/or co-polymers incorporating polyfluorene segments, PPVs and/or related copolymers (see, for example, PCT/WO90/13148). Alternative materials include organic molecular light-emitting materials, e.g. tris(8-hydroxyquinoline)aluminium (Alq3) (see, for example, U.S. Pat. No. 4,539,507, the contents of which are incorporated herein by reference), or any other small sublimed molecule or conjugated polymer electroluminescent material as known in the prior art (see, for example, N. C. Greenham and R. H. Friend, Solid State Physics (Academic Press, San Diego, 1995) Vol. 49, pp 1-149). The light emitted by the device may be inside or outside the visible spectral range (400-800 nm). In the latter case materials such as LDS-821 (A. Dodabalapur et al., IEEE J. Selected Topics in Quantum Electronics 4, 67 (1998)) may be used.
The light-emissive element preferably comprises a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes). There is preferably a region (suitably in the form of a layer) of light emissive material (suitably with other layers to improve performance) between the electrodes. The cathode preferably has a work function of less than 3.5 eV if a cathode or greater than 4.0 eV if a cathode. The material of the cathode is suitably a metal or alloy. Preferred materials include Sm, Yb, Tb, Ca, Ba, Li or alloys of such elements with each other and/or with other metals such as Al. The anode preferably has a work function of greater than 4.0 eV and preferably greater than 4.5 eV. Preferred materials include conductive oxides (such as ITO and tin oxide) and gold. Preferably one of the electrodes is light-transmissive to allow light generated in the device to escape. In one preferred configuration the output electrode of the switching element is also one electrode (the anode or the cathode) of the light-emissive element.
The said integrated circuit device is suitably made up of layers. Preferably the switching element is provided by a first layer and the second circuit element is provided by a second layer, so that the two elements are not co-planar. There is suitably an insulating layer between the first layer and the second layer, and there may be electrically conductive interconnects that pass through the insulating layer to electrically connect the switching element and the second circuit element. The terms xe2x80x9cfirst layerxe2x80x9d and xe2x80x9csecond layerxe2x80x9d do not imply that the layers are deposited in any specific order: either layer could be deposited first.
There is preferably an insulating layer formed either directly or indirectly on top of the semiconductive polymer. The insulating layer may have a low electrical conductivity. It may be an inorganic dielectric such as SiOx, MgF or an organic dielectric such as PMMA, polyimide, or poly-vinylphenol (PVP). The insulating layer may be deposited by vacuum deposition techniques or solution processing. It may consist of a composite or a layered structure with several different components of different functionalities. The insulating layer may comprise a material that is capable of attracting residual dopants from the semiconductive polymer. The insulating layer preferably spaces subsequently deposited layers of the second circuit element from the semiconductive polymer. There may be means for electrical interconnection through the insulating layer, such as via holes preferably containing electrically conductive material. The insulating layer may act so as to at least partially encapsulate the semiconductor polymer. The insulating layer is preferably in contact with the semiconductive polymer, most preferably at a location between the input and output electrodes. The insulating layer is suitably of a material that tends to attract dopants such as oxygen from the semiconductive polymer. Oxygen acts as an unintentional dopant for most semiconductive polymers and reduces the ON/OFF current ratio of the switching element. One possibility is for the insulating layer to be of a silicon oxide, especially a sub-stoichiometric silicon oxide (SiOx, x less than 2).
Where present, the insulating layer may provide one or more further advantageous features. The insulating layer may have different wetting properties to an adjacent conductive portion of the device (e.g. an electrode) to allow differential wetting effects to be used to guide the deposition of material in a desired location. The wetting properties of the surface of the insulating layer may be engineered to attract or repel subsequently deposited polymer material (suitably solution processable polymer material) and/or to enable the fabrication of a multilayer structure. The insulating layer and/or the electrodes on top of the semiconductive polymer may be used to overcome the solvent compatibility and surface wetting compatibility problems which arise when subsequent layers are deposited. By suitable choice of the insulating layer and its surface properties subsequent layers can be deposited from solvents which would otherwise dissolve or not wet the semiconductive polymer and/or layers underneath. The insulating layer may be capable of attracting dopants such as oxygen to inhibit degradation of the device. The insulating layer may assist in resisting delamination or other forms of mechanical failure due, for example, to differential thermal expansion of the materials on either side of it. The insulating layer may be used to planarize the underlying structure. It may have a composite or layered structure such that the interfaces with the first and second circuit elements may have different and optimum properties such as strong adhesion, or good wetting properties.
The switching element is preferably part of a control circuit for the second circuit element, such as an optical data transmission device or an active matrix control circuit for a light-emissive element in a visual display.
It is preferred that in the finished device the semiconductive polymer material is, at least in part, ordered as between polymer chains. One preferred form of ordering is for the polymer chains and/or the direction of strongest electronic overlap between adjacent polymer chains (xcfx80xe2x80x94xcfx80 stacking direction) to be predominantly in a plane that also includes a direction generally between the input and output electrodes. The polymer preferably has a conjugated backbone. The ordering may take the form of at least partial phase separation of the polymer. The polymer material is suitably a material that has a tendency to self-organise, preferably when dissolved in a suitable solvent. The polymer suitably has substituents either in or pendent from its backbone which promote ordering of adjacent polymer chains. The polymer may have hydrophobic side-chains. The ordering, whether self-ordering or imposed ordering is preferably into a lamellar structure, most preferably having alternating layers of certain characteristicsxe2x80x94for instance alternating conjugated (partially or fully conjugated) and (at least substantially) non-conjugated layers and/or alternating main-chain and side-chain layers. The lamellae are preferentially in a plane that also includes a direction generally between the input and output electrodes.
One preferred form of the semiconductive polymer material is a backbone comprising thiophene groups with alkyl side-chains of a length in the range from C3 to C12. Poly-hexylthiophene is especially preferred.
More of the components of the device may be of organic materials. One or more (and most preferably all) of the electrodes may comprise an organic material, suitable a conductive material such as polyaniline or poly-ethylene-dioxythiophene, PEDOT doped with polystyrenesulphonic acid (PSS) (Carter et al., Appl. Phys. Lett. 70, 2067 (1997)). One or more (and most preferably all) of the insulating layers contained in the device may be an organic insulator such as polymethylmethacrylate (PMMA) (see G. Horowitz et al., Adv. Mat. 8, 52 (1996)). The whole device structure may be formed on an organic substrate.
The semiconductive polymer is preferably deposited on to a smooth surface. It is therefore preferred that the input and output electrodes are deposited over the semiconductive polymer and/or that the switchable region is in the form of a layer located between the switching electrode and the input and output electrodes.
The step of depositing the semiconductive polymer preferably comprises dissolving the polymer in a solvent in which the polymer has a tendency to self-organise and/or the step of coating the polymer from a solvent in which the polymer has a tendency to self-organise. In either case, the solvent may be chloroform. If the polymer is, for example, poly-hexylthiophene then the concentration of the polymer in the solvent may be in the range from 6 to 20, more preferably 11 to 15 and most preferably around 13 mg of polymer in 1 ml of solvent. The method of coating is suitably spin-coating, but other processes such as ink-jet printing may be suitable.
The semiconductive polymer is preferably in the form of a layer, and the thickness of the layer is suitably in the range from 200 xc3x85 to 1000 xc3x85, preferably 400 xc3x85 to 600 xc3x85, most preferably around 500 xc3x85.
The step of depositing the semiconductive polymer is preferably performed in an inert atmosphere, for example nitrogen or argon.
The step of depositing the semiconductive polymer comprises preparing a substrate which may consist of a sequence of layers to promote the ordering of the polymer and depositing the polymer onto the substrate. This suitably results in preferential alignment of the polymer chains and/or the direction of strongest electronic overlap between adjacent polymer chains (xcfx80xe2x80x94xcfx80 stacking direction) parallel with the surface of the substrate. The step of preparing the substrate comprises making the surface of the substrate more hydrophobic and/or removing water from the surface and/or treating the surface with a silylating agent. The substrate is preferably maintained in an inert atmosphere between such treatment and deposition of the semiconductive polymer.
A method according to the present invention preferably includes the step of integrating an electrooptical device with the electronic device that incorporates the semiconductive polymer. The electrooptical device is suitably formed directly or indirectly on top of the electronic device, so that the two devices are in a stacked rather than a co-planar arrangement.
Alternative methods of deposition for the polymer/copolymer material(s) include spin-, blade-, meniscus-, dip-coating, self-assembly, ink-jet printing, etc. The polymer material(s) are preferably solution-processible. Layers of small molecule materials can be deposited by vacuum sublimation, etc.
The different layers of the device may be patterned laterally by a suitable technique such as shadow-mask evaporation, ink-jet printing, contact printing, photolithography, etc.
In general, the electronic device is preferably a switching device, more preferably a transistor.